Internal combustion engines (i.e., those having an intake stroke, a compression stroke, a power stroke, and an exhaust stroke, either as separate strokes (four-stroke) or combined (two-stroke) events) may be divided into two general types: spark-ignited and compression-ignited (e.g., diesel).
Spark-ignited engines and compression-ignited engines each have distinct advantages and disadvantages. For example, as versus compression-ignited engines, spark-ignited engines are generally less expensive to produce, have a greater power density (i.e., horsepower produced per volume of cylinder displacement), and are usually supplied with stoichiometric air/fuel ratios that produce relatively low levels of pollutant emissions. The pollutants that are produced by spark-ignited engines run with stoichiometric air/fuel ratios can also be further reduced to currently acceptable levels by utilizing the post-combustion catalytic converter technology available today.
However, the stoichiometric air/fuel ratios required by spark-ignited engines are generally much richer as compared to the air/fuel ratios utilized in compression-ignited (e.g., diesel) engines. Whereas a spark-ignited engine may run on an air/fuel ratio in the ratio of 20:1, a compression-ignited engine may utilize a much higher air/fuel ratio in the range of 40:1 or 50:1. Therefore, compression-ignited engines generally exhibit better fuel economy.
Compression-ignited engines, which run on such lean air/fuel mixtures and do not operate nearly as close to stoichiometric conditions as spark-ignited engines, tend to produce a higher rate of undesirable emission pollutants. Moreover, the emission pollutants that are produced by compression-ignited engines are not nearly as amenable to treatment by the post-combustion catalytic technology currently available, as are the pollutants produced by spark-ignited engines. Chief among the pollutants produced by combustion-ignition engines are nitrogen-containing compounds (i.e., NOX). Such nitrogen-containing compounds result, at least in part, from the high temperatures produced during compression-ignition. Soot is another pollutant produced in greater quantities during combustion-ignition, and arises primarily from the manner in which fuel droplets sprayed into the hot compressed air burn.
Additionally, as noted above, compression-ignition engines tend to have a significantly lower “power density” as compared to spark-ignited engines. For example, while a high performance spark-ignited engine may produce in the range of 60 horsepower per liter of engine displacement, a compression-ignited engine may produce only in the range of about 10 horsepower per liter of engine displacement. A need exists for improvements.